Broadcast frequency spectrum is a resource in which the demand, compared to the spectrum availability, is high because there are many other forms of broadcast communication, such as television, radio, and wireless communication services, that also require bandwidth for transmitting data. The use of the spectrum is therefore highly regulated by the Federal Communications Commission (“FCC”).
Digital communication is becoming a more common method for broadcast communication. In general, digital communication allows for the delivery of communication services that are of a better quality than counterpart analog methods of communication and can be made to be faster and more efficient than analog communication.
Digital Audio Broadcasting (DAB) is among the various services that require bandwidth. DAB is a digital method of transmitting audio signals and data services to radio receivers. DAB is superior to existing methods for transmitting analog AM and FM audio signals because DAB delivers audio signals that are near-CD quality. As a result, there is a demand for DAB and a consequent need for bandwidth. Because there is a desire by broadcasters to introduce DAB without significant increases in costs, broadcasters are seeking to introduce DAB without the need for new spectrum allocations.
One approach to this problem has been to transmit digital signals simultaneously with FM signals in the same band as the FM signal on the same radio station dial positions, i.e., the same channel, as the analog FM signals being transmitted. This is generally referred to as Hybrid In-Band, On-Channel (IBOC) FM combining. The resulting combined signal has a digital signal component and an FM signal component. As shown in FIG. 1, the digital components of the hybrid signal are spaced around the existing FM signal. The Hybrid IBOC DAB approach involves combining the digital signal and the FM signal after they have been separately amplified. Generally, the signals are combined after they have each been amplified because, in most cases, the FM transmission system is already in existence, and includes amplification of the FM signal.
An advantage of Hybrid IBOC DAB is that the combined FM and digital signals still fit within the FM FCC mask for the FM signal being combined. The FCC mask defines the boundaries for transmitting the existing FM signal. As shown in FIG. 1, the digital component of the signal is of relatively low power as compared to the FM component of the signal and fits within the FCC mask. Confining the combined signal within the FCC mask is described by Kroeger and Cammarata in “Robust Modem and Coding Techniques for FM Hybrid IBOC DAB,” IEEE Broadcast Symposium 1997. Another advantage of Hybrid IBOC DAB, for signal transmission, is its efficiency over corresponding analog methods for signal transmission. In FM IBOC combining, error correction and data compression are performed in the generation of the digital broadcast signal and do not have to be separately performed. Thus, part of the efficiency of Hybrid IBOC DAB systems can be attributed to the reduction in signal transmission steps over the corresponding analog methods for signal transmission.
A disadvantage of the Hybrid IBOC DAB approach is the difficulty associated with keeping the signals isolated from each other, in such a way as to avoid intermodulation product generation in the transmitters. Intermodulation products are radio-frequency (RF) products that typically occur within the bandwidth of the existing FM signal (in-band), and also in adjacent channels (out-of-band). In-band intermodulation products generally cause distortion in the combined signal, and out-of-band intermodulation products generally affect the performance of other signals in adjacent channels.
The use of filters is one approach to reducing intermodulation products. However, utilizing filters in combining systems to provide isolation between the signals would be difficult because it would require a system having a large number of high quality factor (Q) sections. Thus, filters that pass signals within a very narrow bandwidth would be needed. Essentially, a “brickwall” filter would be needed, in which, for the most part, there is no transition bandwidth region between the signals which would be allowed to pass through the filter. Therefore, there would be no range of frequencies between the passband and the stopband. This approach would be difficult and costly to implement.
Another approach to reducing the generation of intermodulation products involves a combining system that utilizes a single −10 dB quadrature hybrid coupler, as shown in FIG. 2. The digital signal loses power, approximately 90%, to the reject load. This is typically not significant because the power required for the digital signal is low.
A disadvantage to this, however, is the loss of power suffered by the FM signal. The FM signal loses about 0.46 dB to the reject load when the FM signal passes through the −10 dB quadrature coupler. The insertion loss of the FM signal is significant although the amount of power lost by the FM signal is low compared to the amount of power lost by the digital signal. As a result of power lost by the FM signal, a larger transmitter may be required to compensate for the loss of power of the FM signal, so that the FM or combined signal is able to achieve the assigned Effective Radiated Power (ERP).
Although a system utilizing a single −10 dB quadrature hybrid coupler does provide for the isolation of the signals during transmission, it is inefficient because of the power lost in the reject loads.
Accordingly, it is desirable to provide an efficient system for transmitting digital signals with FM signals that maintains isolation between the two different signals during transmission and eliminates the loss that occurs when FM signals and digital signals are combined.